Generic ExtensibleCollectionType algorithm plus a default Array version

ExtensibleCollectionPlusDefault.swift

// This algorithm returns a generic placeholder
// that can be any kind of extensible collection:
func mapSome
  <S: SequenceType, C: ExtensibleCollectionType>
  (source: S, transform: (S.Generator.Element)->C.Generator.Element?) -> C {
    var result = C()
    for x in source {
        if let y = transform(x) {
            result.append(y)
        }
    }
    return result
}

// usage example:
let strings = ["1","2","3"]
let toInt = { (s: String) -> Int? in s.toInt() }

let a: Array = mapSome(strings, toInt)
let b: ContiguousArray = mapSome(strings, toInt)

// however, it's annoying to have to specify a type
// every time when you probably want an array.
// cue obscure Swift compiler error...
let c = mapSome(strings, toInt)

// If you write a second version that returns an array,
// you can call the more general version for implementation:
func mapSome<S: SequenceType,U>(source: S, transform: (S.Generator.Element)->U?)->[U] {
    // just calls the more generalized version
    // (works because here, the return type 
    // is now of a specific type, an Array)
    return mapSome(source, transform)
}

// and now, if you don't specify, you get an array by default:
let d = mapSome(strings, toInt)